Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. In color flow imaging, the flow of blood or movement of tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. The Doppler shift may be displayed using different colors to represent speed and direction of flow. The color flow mode displays hundreds of adjacent sample volumes simultaneously, all color-coded to represent each sample volume's velocity. The color flow image may be superimposed on the B-mode image.
The present invention is incorporated in an ultrasound imaging system consisting of four main subsystems: a beamformer 2 (see FIG. 1), processor subsystem 4, a scan converter/display controller 6 and a master controller 8. System control is centered in master controller 8, which accepts operator inputs through an operator interface (not shown) and in turn controls the various subsystems. The master controller also generates the system timing and control signals which are distributed via a system control bus 10 and a scan control bus (not shown).
The main data path begins with the digitized RF inputs to the beamformer from the transducer. The beamformer outputs two summed digital baseband receive beams. The baseband data is input to B-mode processor 4A and color flow processor 4B, where it is processed according to the acquisition mode and output as processed acoustic vector (beam) data to the scan converter/ display processor 6. The scan converter/display processor 6 accepts the processed acoustic data and outputs the video display signals for the image in a raster scan format to a color monitor 12. The scan converter/display controller 6, in cooperation with master controller 8, also formats multiple images for display, display annotation, graphics overlays and replay of cine loops and recorded timeline data.
The B-mode processor 4A converts the baseband data from the beamformer into a log-compressed version of the signal envelope. The B function images the time-varying amplitude of the envelope of the signal as a grey scale using an 8-bit output for each pixel. The envelope of a baseband signal is the magnitude of the vector which the baseband data represent.
The frequency of sound waves reflecting from the inside of blood vessels, heart cavities, etc. is shifted in proportion to the velocity of the blood cells: positively shifted for cells moving towards the transducer and negatively for those moving away. The color flow (CF) processor 4B is used to provide a real-time two-dimensional image of blood velocity in the imaging plane. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate. Instead of measuring the Doppler spectrum at one range gate in the image, mean blood velocity from multiple vector positions and multiple range gates along each vector are calculated, and a two-dimensional image is made from this information. The structure and operation of a color flow processor are disclosed in U.S. Pat. No. 5,524,629, the contents of which are incorporated by reference herein.
The color flow processor produces velocity (8 bits), variance (turbulence) (4 bits) and power (8 bits) signals. The operator selects whether the velocity and variance or the power are output to the scan converter. The output signal is input to a chrominance control look-up table which resides in the video processor 22. Each address in the look-up table stores 24 bits. For each pixel in the image to be produced, 8 bits control the intensity of red, 8 bits control the intensity of green and 8 bits control the intensity of blue. These bit patterns are preselected such that as the flow velocity changes in direction or magnitude, the color of the pixel at each location is changed. For example, flow toward the transducer is indicated as red and flow away from the transducer is indicated as blue. The faster the flow, the brighter the color.
The acoustic line memories 14A and 14B of the scan converter/display controller 6 respectively accept processed digital data from processors 4A and 4B and perform the coordinate transformation of the color flow and B-mode data from polar coordinate (R-.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data stored in X-Y display memory 18. In the B-mode, intensity data is stored X-Y display memory 18, each address storing three 8-bit pixels. Alternatively, in the color flow mode, data is stored in memory as follows: intensity data (8 bits), velocity or power data (8 bits) and variance (turbulence) data (4 bits).
A multiplicity of successive frames of color flow or B-mode data are stored in a cine memory 24 on a first-in, first out basis. The cine memory is like a circular image buffer that runs in the background, continually capturing image data that is displayed in real time to the user. When the user freezes the system, the user has the capability to view image data previously captured in cine memory. The graphics data for producing graphics overlays on the displayed image is generated and stored in the timeline/graphics processor and display memory 20. The video processor 22 multiplexes between the graphics data, image data, and timeline data to generate the final video output in a raster scan format on video monitor 12. Additionally it provides for various greyscale and color maps as well as combining the greyscale and color images.
A conventional ultrasound imaging system collects B-mode or color flow mode images in cine memory 24 on a continuous basis. The cine memory 24 provides resident digital image storage for single image review and multiple image loop review and various control functions. The region of interest displayed during single-image cine replay is that used during the image's acquisition. The cine memory also acts as a buffer for transfer of images to digital archival devices via the master controller 8.
In conventional diagnostic ultrasound imaging systems, the velocity color flow mode suffers from inherent limitations due to the nature of a sampled data system and the velocity estimator. In particular, the velocity mode suffers from aliasing where flow velocities exceeding PRF/2 are wrapped into and cannot be distinguished from other velocities. In addition, the wide variety of flow states in the human body which must be simultaneously imaged, such as slow-moving weak flow in the kidney and high-velocity strong flow in the aorta, prevent the system designer from optimizing the system a priori, and require the development of user optimization and/or adaptive optimization tools.
Two-dimensional ultrasound images are often hard to interpret due to the inability of the observer to visualize the two-dimensional representation of the anatomy being scanned. However, if the ultrasound probe is swept over an area of interest and two-dimensional images are accumulated to form a three-dimensional volume, the anatomy becomes much easier to visualize for both the trained and untrained observer. In particular, three-dimensional ultrasound imaging of moving fluid or tissue would be advantageous.
However, in three-dimensional renderings of velocity data, the projection algorithm is extremely sensitive to aliasing between two-dimensional frames. This is especially true when a maximum pixel projection algorithm is used because aliased data in one frame will often have a higher absolute velocity than the data in an adjacent frame without aliasing. Three-dimensional renderings accentuate the effect of aliasing. Furthermore, pulsatility in vessels from the cardiac cycle creates multiple images or dropouts in large vessels which provide inaccurate three-dimensional renderings.
In a conventional ultrasound imaging system, wall filters and compression curves are applied to the beamformed color flow data, positive and negative velocities are estimated, post-processing such as frame averaging and thresholding are applied, and then the data is displayed using a non-symmetric color map whereby positive and negative flow states are represented by different colors and/or intensities. Aliasing in the velocity data shows up as rapid color transitions across the aliasing boundary which do not represent true flow states and may be extraneous information and a distraction to the user.
Furthermore, in a conventional ultrasound imaging system, frame averaging of velocity data must consider the sign and magnitude of the data to determine whether the flow has aliased, and then adjust for the aliasing in the algorithm. Frame averaging across the alias boundary is difficult and an algorithm which must handle aliasing will have sub-optimal performance on non-aliased data.